In its most usual form, a robot or manipulator comprises a series of arm segments terminating in a wrist assembly. The wrist assembly supports a face plate to which an appropriate tool is affixed. The nature of the tool will depend upon the work to be performed by the manipulator.
The manipulator is provided with a control. The control is programmed to cause the manipulator to move the tool along a programmed path of travel and to execute programmed functions associated with programmed locations along that path of travel.
In order for the manipulator to perform operations correctly, it is necessary that the manipulator move the designated center point of the tool accurately along the programmed path of travel. Some operations require greater accuracy than others. If the manipulator supported tool is a welding torch, for example, it is desirable, and generally necessary, that the tool center point (the tip of the welding tool) be moved along the seam to be welded with a high degree of accuracy.
The tool dimension with which the present invention is concerned may be defined as the distance between the center point of the manipulator wrist and the designated center point of the tool. This is a vector distance made up of components L, B and A, as will be described hereinafter.
A program defining a cycle of operation of the manipulator consists of coordinates of locations defining end points of manipulator motion. So long as the manipulator and the tool used to teach the program are used to execute the program in the same location in which it is taught, the operation will be performed with the accuracy inherent in the manipulator and control combination. If, however, it is necessary to perform the program relative to a shifted workpiece or using a replacement tool, or if the program to be executed is not created using the manipulator and tool with which it is to be executed, then inaccuracy in the specification of tool dimensions can result in inaccuracy in the performance of the cycle of operation. This is a consequence of the use of the operator specified tool dimensions in the calculations involved in effecting manipulator motion. In the case of program alignment for a shifted workpiece, any difference between the operator specified tool dimensions and the actual tool dimensions used in performing alignment can result in creating incorrect coordinate corrections for aligning the program with the actual workpiece location. Likewise, a difference between the actual tool dimensions and the operator specified tool dimensions at the time of program teaching can result in production of location coordinates which are not properly compensated by new tool dimensions. The need for accurate tool dimensions imposes a burden on the manipulator user to ascertain those dimensions prior to program creation, in the case of teaching, or prior to program execution, in the case of programs created from a source other than the manipulator with which the program is to be executed.
Heretofore, prior art workers have generally determined the tool dimensions through the use of a scale or other direct measurement means. By virtue of the complex geometry of some manipulator tools, such measurements frequently yield rough approximations of the true tool dimensions. This, in turn, leads to undesirable or even unacceptable tool movement during the manipulator's controlled path operation.
The present invention is based upon the development of a procedure for more accurately determining the correct dimensions of any three dimensional manipulator tool. The procedure is in part empirical and in part analytical and can readily be performed by a manipulator operator of ordinary skill in the art.